Transfer-path analysis (TPA) is a reliable and effective diagnostic tool for determining the dominant vibration transfer paths from the actively vibrating components to the connected passive substructures in complex assemblies. Conventional and component-based TPA approaches achieve this by estimating a set of forces that replicate the operational responses on the passive side of the assembly, requiring separate measurements of the transfer-path admittance and the operational responses, followed by an indirect estimation of the interface forces. This demands significant measurement effort, especially when only the dominant transfer paths are desired. Operational transfer-path analysis (OTPA) overcomes this by identifying transfer-path contributions solely from operational response measurements. However, OTPA is susceptible to measurement errors as minor inaccuracies can result in discrepancies regarding transfer-path characterization. This is especially evident when poor placement of the sensors results in similar response measurements from multiple channels, introducing redundancy and amplifying measurement noise. This is typically resolved using regularization techniques (e.g., singular-value truncation and Tikhonov regularization) that promote vibration transfer related to dominant singular vectors. As an alternative, this paper explores the benefits of using established reduction-based approaches from dynamic substructuring within OTPA. Measured responses are projected onto different dynamic sub-spaces that include the dominant dynamic behavior of the interface between the active and passive sides (i.e., dominant interface modes). In this way, only the vibration transfer related to the interface modes included in the reduction step is evaluated, leaving stiff modes obscured by noise unobserved. This paper proposes using interface-deformation modes and physical modes, demonstrating their feasibility via various experimental setups and comparing them to standard OTPA.